A mass spectrometer is an analytical instrument which is capable of identifying an unknown material. The identification process begins by ionizing the unknown material. The ions are next separated by the mass to charge ratio. The ions are then detected by an electron multiplier which amplifies the weak signal produced by the ions. The amplified signals are then recorded by a computer or other instrument as a series of mass peaks. By comparing these mass peaks to those recorded in a library, the unknown material can be identified with a high degree of accuracy.
MALDI is a form of photo-ionization that has become a popular ionization technique for organic and biological compounds because the resulting series of ions is rich in structural information about the compound. In the MALDI process, the material to be analyzed (the analyte) is mixed with a matrix material in order to enhance the absorption of the energy from the photon source. The matrix material is typically a form of salt. The mixture of the analyte material and the matrix material is then spotted onto a target referred to as a MALDI Plate or MALDI Target. The spots are typically deposited in rows and columns by a robot. Each position corresponds to a sample number. Dozens of samples can be loaded onto a single sample plate, which is a significant productivity advantage. The spots are then dried of all solvents and the plate is loaded into the mass spectrometer for analysis. Loading and unloading of the mass spectrometer is also automated in modern machines.
FIG. 1 schematically illustrates the structure and operation of a MALDI time-of-flight mass spectrometer. The mass spectrometer 10 has an ionization section 12, an ion drift chamber 14, and a detection section 16. The ionization section 12 includes a target plate 18 on which at least one spot sample 20 is deposited and a pusher plate assembly 22 which is connected to a voltage source (not shown). A laser 24, preferably a nitrogen laser, is disposed for directing a pulsed laser beam 26 onto the spot sample 20. The detection section 16 includes a detector 28 which is preferably a microchannel plate-type ion detector.
In operation, the nitrogen laser 24 is operated to aim at a fraction of single spot. The laser is fired in a short burst which briefly exposes the selected spot sample to the intense light energy. The matrix material is specifically chosen to be able to absorb the energy from the laser pulse. As the matrix absorbs the laser energy, a hypersonic explosion occurs which causes the analyte material to fractionate and ionize.
The resulting ions are then pushed out into a field free region in the drift chamber 14 through the application of a high voltage pulse to the pusher plate assembly 22. The ions travel toward the detection section 16, with the lower mass ions reaching the detector 28 first and the highest mass ions arriving last. Each time a group of ions with the same mass reach the detector, a very fast voltage pulse is produced by the detector which can be recorded.
In the time-of-flight mass spectrometer 10, the exact mass of an ion can be determined, and therefore identified, by precisely recording the amount of time it takes for the ion to travel through the field free region. This is usually done by solving the equation KE=½ mv2.
The accuracy of a MALDI time-of-flight mass spectrometer depends not only on the precise recording of the ion arrival times, but also on the assumption that all the ions of a given mass arrive at nearly the same time. In practice this latter assumption is seldom achieved. Modern ion detectors have a temporal response of less than 400 picoseconds. However, the time window in which ions of the same mass arrive at the detector can be thousands of times longer than the response time. Although there are many contributing factors, one of the largest contributors is the spatial distribution of the ions immediately after the hypersonic explosion.
The analyte-matrix spot samples for MALDI analysis are typically deposited on a polished metal plate in rows and columns. When the laser radiation impinges on the matrix material, the resulting hypersonic explosion sends the ions out in all directions with significant velocity. FIG. 2 illustrates this effect. The ion cloud 30 is large and interdispersed with ions of very different masses. Because ions of like masses begin their journey from different locations within the ion cloud source, travel times will differ in proportion to the distance traveled. The differences in travel time are manifested as time jitter which serves to degrade the mass resolution.